1. Technical Field
This invention relates generally to a method for using a variable valve actuation system to control the temperature and space velocity of exhaust gases delivered to an aftertreatment system of a Diesel engine, and more particularly to a method for using a variable valve actuation system to control exhaust gas temperature during regeneration of exhaust gas aftertreatment devices.
2. Background Art
Worldwide emissions regulations slated for introduction during the next five to ten years will require that Diesel engines be equipped with some form of exhaust aftertreatment device, forcing the Diesel industry into utilizing catalyst and emissions “trap” technologies not widely used heretofore for Diesel engines. For optimum efficiency, these technologies, e.g., lean NOx catalysts, lean NOx traps, and particulate traps, require operating temperatures outside of the temperature range available for a significant portion of a Diesel engine operating range.
Several approaches have been proposed to control exhaust gas temperature in spark-ignition engines. For example, U.S. Pat. No. 6,029,626 granted Feb. 29, 2000 to Claus Bruestle for ULEV CONCEPT FOR HIGH-PERFORMANCE ENGINES proposes using variable valve timing to reduce unburned hydrocarbon emissions. Unburned hydrocarbon emissions are a pollutant commonly associated with spark ignited engines. U.S. Pat. No. 5,398,502 granted Mar. 21, 1995 to Kenzo Watanabe for a SYSTEM FOR CONTROLLING A VALVE MECHANISM FOR AN INTERNAL COMBUSTION ENGINE is also directed to exhaust gas temperature regulation in spark-ignition engines. More specifically, U.S. Pat. No. 5,398,502 proposes the use of variable valve timing to control exhaust temperature as a strategy to increase exhaust gas temperature for fast catalytic converter lightoff during engine cold-starts. This patent further proposes exhaust gas temperature control when it is deemed that catalyst damage may occur due to over-heating.
As noted above, because of tightening emissions regulations worldwide, future Diesel engine powered vehicles will have to use one or more types of exhaust gas aftertreatment devices, including particulate filters, lean NOx adsorbers or traps, oxidation and/or other catalysts. Some aftertreatment devices must be regenerated to maintain their pollutant conversion efficiency. This is typically achieved by operating the Diesel engine in a mode that produces exhaust gas conditions that regenerate the aftertreatment device by temporarily raising its temperature or by changing the composition of the exhaust gases flowing through it. However, when regeneration is required under some engine operating conditions (typically very high load conditions), the exhaust gas temperature can be so high as to damage some of the engine components, such as exhaust valves and turbocharger, or the aftertreatment devices themselves.
The present invention is directed to overcoming the problems associated with protecting temperature sensitive engine components and aftertreatment devices during regeneration. It is desirable to have a method for controlling the temperature and/or mass flow rate of exhaust gases during temporary periods of operation required to regenerate one or more aftertreatment devices. Moreover, it is desirable to have such a method that advantageously uses variable valve actuation to control the temperature and mass flow rate of exhaust gases introduced into to the exhaust stream.